DE4243144A1 - Objective for an FT Raman microscope - Google Patents

Description

The invention relates to a Raman microscope, in particular for a Fourier transform (FT) spectrometer, with a lens for magnifying image of a punctiform area on a Surface of a test sample, with means for irradiating Laser radiation on the punctiform area and by means for the detection of the emitted Raman radiation.  

Such a Raman microscope is known for example from the Conference contribution "Holographic Optical Components for Laser Spectroscopy Applications "by Harry Owen, distributed at the Conference "Holographics International ′92", 26-29. July 1992, London, England.

Raman scattering is a very weak process in which a photon interacts with a molecule and is scattered inelastically. The photon loses or gains energy in this process, which is expressed in a frequency shift of the scattered photon. This frequency shift corresponds to rotational, vibration or electronic state transitions of the molecule on which the photon is scattered. For vibration transitions, the scattering probability is approximately 10 ^-7 . In contrast, the elastic scattering process known as Rayleigh scattering, in which no energy transfer takes place during the interaction between photon and molecule, is considerably stronger. Raman spectroscopy has been developed into a very sensitive and meaningful method for the determination of chemical and molecular structures in liquids, solids and on surfaces. It provides complementary statements to the usual infrared (IR) spectroscopy, since molecular transitions are possible in Raman processes, which are prohibited in the processes of IR spectroscopy.

In recent years, near infrared (NIR) Lasers, such as neodymium-YAG lasers for the Raman Spectroscopy, especially in Fourier transform (FT) spectro meters used. Such an FT-IR spectrometer is an example wise in the company font "IFS 66" from Bruker Analytische Meßtechnik GmbH described. The classic Raman  Experiments extremely disruptive fluorescent light in the visible The area can be excited by using NIR lasers are essentially avoided by Raman processes.

For the examination of very small or inhomogeneous samples with the help of Raman spectroscopy in the Raman spectrometer Microscopes with appropriate lenses for magnifying Illustration of a punctiform area on the surface of the Measurement sample used. This usually from optical Lens-built microscope objective is cited in both above described publications. Such a commercial available, optimized for the visible wavelength range Glass lens that is used for the injection of focused laser light on the sample surface and to record the emitted Raman Spectrum is used, but has the disadvantage of a chroma tables aberration that leads to distortion of the recorded Spectra leads. Therefore, these glass lenses are only in one very limited optical range can be used.

Against the chromatic aberration in a certain wave conventional glass lenses are covered by coating anti-reflective coating. This anti-reflective coating only works in sight wavelength range, while the lens in the NIR range leads to aberrations. An anti-reflective coating would also be and correction of the known glass lenses for the IR range in principle possible. Such lenses would be in the Manufacturing very expensive, and the corrective effect would even for a small section of the IR wavelengths area become effective. Therefore, anti-glare IR glass lenses hardly available in normal trade and very expensive.

The object of the present invention is therefore to provide a Raman To introduce microscope of the type described above that on simple way from commercially available optical Components is built up and no chromatic aberration points.

According to the invention this object is achieved in that the Laser radiation in the infrared (IR) -, preferably in the near infrared (NIR) range, and that the lens is a Cassegrain game gel lens with a rotationally symmetrical to an optical Axial convex mirror and one also for opti axis of the lens rotationally symmetrical concave mirror , the focal point of the optical axis Concave mirror with the punctiform area on the surface the test sample agrees. The Cassegrain mirror used Objectively, for example from DE-OS 33 03 140 is known per se, is achromatic and therefore leads to no distortions in the NIR Raman spectrum. Although the inventions In retrospect, problem solving is amazingly simple no Raman microscope is known the one where a Cassegrain lens was used.

In one embodiment of the invention, the means for Irradiation of the laser radiation designed so that the laser radiation first onto the reflecting surface of the convex mirror hits, with the essential part of the laser radiation with Ab stood from the optical axis of the Cassegrain mirror lens is irradiated. By the arrangement of the means for irradiation len of the laser radiation on the side of the Cassegrain mirror lenses do not cause space problems visually the positioning of the coupling mirror (s). Due to the radiation of at least a significant part  of the laser light with a lateral offset from the optical axis There is little or no laser light in the lens a detector leading beam path from the lens emerging Raman scattered light.

A preferred way of coupling is that the laser radiation via a semitransparent plane mirror, that on the optical axis at a distance from the Cassegrain mirror objectively on the opposite side of the test sample is arranged at an angle to the optical axis, on the Kon Vex mirror is directed, the laser radiation being so diverse is widened that it is symmetrical to the optical Axis of the conical beam strikes the convex mirror, and wherein the cone angle is chosen so that the laser radiation after going through the Cassegrain mirror lens to the point shaped area is focused.

This will make the Cassegrain mirror lens from the incident Laser radiation fully illuminated and the latter ideal for Raman excitation on the sample surface exploited.

In a preferred development of this embodiment the semi-transparent plane mirror for Raman radiation in one observed wavelength range transmitting and for the radiated NIR laser radiation reflective. In this way can the coupled laser radiation completely and without Losses can be used for the Raman measurement.

In another embodiment, the coupling is thereby causes the laser radiation as a parallel beam with little Beam diameter over a fully reflective plan mirror with a lateral offset to the optical axis and  on the side of the Cassegrain mirror lens facing away from the sample is arranged at such an angle with respect to the opti rule is directed to the convex mirror that the Paral oil bundle after passing through the Cassegrain mirror lens strikes the punctiform area. Even in this arrangement is a reflection of the injected laser light in the direction prevented on the detector arrangement of the spectrometer, wherein however, part of that from the Cassegrain mirror lens emerging Raman radiation by reflecting completely the plan mirror also remains hidden. The injected Laser light, which is a very thin cross-section, High intensity beam can act with this version form most favorably on the lateral surface of the cone, the normally when operating the Cassegrain mirror lens visible light turned on with a certain expansion angle radiates around the convex mirror as completely as possible to illuminate, and that from the mirror lens to a point Area is focused on the sample surface.

In a particularly preferred embodiment, the on Coupling of the laser radiation causes that the means for irradiating the laser radiation on the optical axis between the sample and the Cassegrain mirror lens include a reflective element, with the help of an im essentially parallel to the optical axis Oil-bundled laser beam along the optical axis onto the point-shaped area on the surface of the test sample can be. With this geometrically extremely simple arrangement there is no loss of the laser incident on the sample radiation due to scattering or absorption in the lens. In addition, there is no beam splitter in the beam path of the  Raman microscope is required, which is usually additional Would result in loss of scattered Raman light.

In a particularly advantageous development of this embodiment form, the reflective element has a smaller space expansion across the optical axis than the convex Cassegrain mirror lens mirror. So that's the reflect Gate element in the shadow of the convex mirror, so that the light yield of the Raman light emerging from the sample through the Reflector element is not weakened.

To a higher radiation density of the incident laser light to achieve on the sample surface, the reflective effect Focusing element in an advantageous embodiment, in particular, it focuses on the incident laser radiation the punctiform area on the surface of the test sample. On this way, above all, a better spatial resolution of the FTIR Raman spectrometer achieved.

The focusing reflector element can be at an angle to the optical axis arranged plane mirror and a in Beam path of the laser radiation, preferably in the beam direction include focusing lens arranged in front of the plane mirror.

In contrast, an embodiment is preferred, however of the reflective element preferably at 45 ° to Parabolic mirror arranged optical axis. The bullet the laser beam then takes place at 90 ° to the optical axis. In this advantageous solution, only an optical one Element for beam deflection and focusing of the incident Laser beam required, so that the losses are particularly small being held.  

The invention is described below with reference to the drawing illustrated embodiments described and he purifies. Those to be found in the description and the drawing Features may be in other embodiments of the invention individually, individually or in several combinations Find application.

Show it:

Figure 1a is a schematic diagram of the beam path in a Raman microscope with a glass lens according to the prior art.

FIG. 1b shows a schematic of the beam path by a grain Casse-mirror lens;

Fig. 2 shows the schematic beam path through an inventive Raman microscope with coupling of the laser light from above, the laser beam

• a) strongly bundled and with lateral offset from the optical axis or
• b) greatly expanded to the optimal single cone
  is coupled into the Cassegrain mirror lens;

Fig. 3 shows the schematic beam path through an inventive Raman microscope with laser beam coupling between the Cassegrain mirror lens and the sample, the laser beam

• a) is heavily bundled and has a plan mirror is directed to the sample,  
• b) is initially widened to a focus lens meets and with a plan mirror is focused on the sample, respectively.
• c) is initially expanded and with a Parabolic mirror deflected and put to the test is focused.

In the beam path shown in FIG. 1 a through a conventional Raman microscope, a laser beam 20 is directed from a laser 10 via a semi-transparent or dichroic mirror 6 through a glass lens 30 onto a sample 2 . The 30 backscattered Raman from the sample surface through the glass lens (and Rayleigh) radiation passes through the glass lens 30 is now in the reverse direction is transmitted by the half mirror 6, and after passing through the other, not shown in the drawing, parts of the spectrometer detected in a detector 40 .

FIG. 1b shows schematically the beam path by a Cassegrain mirror objective with a rotationssym to the optical axis 5 metric convex mirror 3 by which a mirror in the lens a falling first diver constricting beam bundle 21 to a rotationally symmetrical also to the optical axis 5 Concave 4 distracted and there as an incident beam tuft 22 is focused on a punctiform area 1 of the measurement sample 2 , which is attached to a base 12 . The scattered light reflected by the punctiform region 1 can leave the Cassegrain mirror objective 3 , 4 in the opposite way and be guided to other spectrometer parts not shown in the drawing and ultimately to a detector (also not shown). Such a device known per se in the prior art for use in IR spectrometers has never been used to investigate Raman radiation.

In the embodiment of the Raman microscope according to the invention shown in FIG. 2 a), the laser beam 20 is emitted from an infrared (IR), preferably a near infrared (NIR) laser 11 via a gel objective 3 on the side of the Cassegrain mirror which is remote from the sample, 4 with lateral offset to the optical axis 5 arranged plane mirror 7 directed by the Cassegrain lens 3 , 4 to the punctiform area 1 on the surface of the measurement sample 2 . The laser beam 20 emerging from the IR laser 11 is bundled in parallel in this exemplary embodiment and has only a small beam diameter. The most fully reflecting plane mirror 7 is arranged so that the laser beam 20 is deflected onto a surface line 23 of the optimal incidence cone in the Cassegrain mirror lens 3 , 4 , from which it then emerges on the partial path 24 and in the punctiform area 1 on the Sample surface hits.

The Raman light reflected by the measurement sample 2 then travels on the partial path 25 into the Cassegrain mirror objective 3 , 4 and leaves it on a partial path 26 in the direction of the spectrometer parts containing a detector, not shown in the drawing.

The advantage of this embodiment that relatively strongly focused laser radiation is directed onto the punctiform area 1 of the measurement sample 2 is offset by the disadvantage that a completely different area is hit on the sample surface with a slight axial offset of the measurement sample 2 parallel to the optical axis 5 .

This disadvantage does not exist in the embodiment shown in FIG. 2 b), in which an already widened, diverging laser beam 20 is directed from the IR laser onto a semitransparent plane mirror 6 , which bundles a beam 21 symmetrically with respect to the optical axis 5 Coupled Cassegrain mirror lens 3 , 4 . The divergence angle of the laser beam 20 and thus the bundle of rays 21 should be chosen so that the bundle of rays 21 coincides with the optimal cone of incidence of the Cassegrain mirror lens 3 , 4 , so that the convex mirror 3 is optimally illuminated. The bundle of rays 22 emerging from the objective is then focused on the punctiform region 1 of the measurement sample 2 as in the arrangement shown in FIG. 1b. The reflected or scattered Raman light takes the reverse beam path through the Cassegrain lens 3 , 4 back to the semitransparent mirror 6 , which is reflective for the incident laser radiation, but is transparent to the Raman radiation to be observed.

With a small axial offset of the test sample 2 parallel to the optical axis 5 , the surface of the test sample 2 is still irradiated around the punctiform area 1 , but then an extended focal spot arises which corresponds to the focal point 1 when the test sample 2 is precisely adjusted.

In the embodiment shown in FIGS . 3a) to 3c), the coupling of the IR laser radiation takes place via a gel lens 3 , 4 arranged between the measurement sample 2 and the Cassegrain mirror 3 , 4 arranged reflector element.

In Fig. 3a), a simple arrangement is shown with a plane mirror 8 on the optical axis between the convex mirror 3 and the test sample. 2 The parallel bundled laser beam 20 from the IR laser 11 strikes in the example shown from the side on the tilted by about 45 ° against the optical axis 5 plan mirror 8 and from this along the optical axis 5 to the punctiform region 1 deflected the surface of the measurement sample 2 . The Raman radiation emitted by the test sample 2 is collected in a hollow cone region 22 'by the Cassegrain mirror objective 3 , 4 and in a cone region 21 ' convergingly passed on to the other parts of the Raman spectrometer, not shown in the drawing. As a result, on the one hand a relatively strongly focused laser beam 20 with a high intensity density can be brought onto the punctiform region 1 , but on the other hand, in contrast to the embodiment shown in FIG he follows.

In the exemplary embodiments shown in FIGS . 3b) and 3c), the laser radiation 20 emerging from the IR laser 11 is not only deflected, but also focused. In Fig. 3b), an arrangement is shown in which from the IR laser 11 is a relatively strong flared parallel beam 20 emerges that has a focusing lens 9 and a as in the embodiment in Fig. 3a) arranged plane mirror 8 to the point-shaped region 1 of the measurement sample 20 is focused. Such an expanded, parallel bundled laser beam 20 has the advantage over a laser beam with a narrow cross-section that it can be relatively large distances without significant divergence. In contrast, the disadvantage of absorbing a certain proportion of the laser light in the lens 9 is hardly significant.

In Fig. 3c), a particularly preferred exemplary embodiment is shown, in which the reflecting element between the Cassegrain mirror objective 3 , 4 and the measurement sample 2 not only deflects the incident, parallel-widened laser beam 20 , but also simultaneously onto the punctiform region 1 the sample 2 focused. This is achieved by a preferably arranged at 45 ° to the optical axis 5 Para bolspiegel 13 .

It is particularly advantageous if the plane mirror 8 or the parabolic mirror 13 has a smaller spatial dimension transverse to the optical axis 5 than the convex mirror 3 of the Casse grain mirror objective 3 , 4 . In this case, the reflective element 8 , 13 is located in the shadow of the convex mirror 3 , so that the Raman light emitted from the surface of the test sample 2 into the hollow cone 22 'is not weakened by the reflective element 8 , 13 .

Claims (10)

1. Raman microscope, in particular for a Fourier transform (FT) spectrometer, with a lens for magnifying imaging of a punctiform area on a surface of a measurement sample, with means for irradiating laser radiation onto the punctiform area and with means for detecting the emitted Raman Radiation, characterized in that the laser radiation is in the infrared (IR), preferably in the near infrared (NIR) range, and in that the objective is a Cassegrain mirror objective with a convex mirror ( 3 ) arranged rotationally symmetrically to an optical axis ( 5 ) and one also to the optical axis ( 5 ) of the lens is rotationally symmetrical concave mirror ( 4 ), the focal point of the concave mirror ( 4 ) lying on the optical axis ( 5 ) corresponding to the punctiform region ( 1 ) on the surface of the measurement sample ( 2 ) . 2. Raman microscope according to claim 1, characterized in that the means for irradiating the laser radiation are formed in such a way that the laser radiation first hits the reflecting surface of the convex mirror ( 3 ), the major part of the laser radiation being at a distance from the optical one Axis ( 5 ) of the Cassegrain mirror lens ( 3 , 4 ) is irradiated. 3. Raman microscope according to claim 2, characterized in that the laser radiation via a semitransparent plane mirror ( 6 ) on the optical axis ( 5 ) at a distance from the Cassegrain mirror lens ( 3 , 4 ) on that of the test sample ( 2 ) the opposite side is arranged at an angle to the optical axis ( 5 ), is directed onto the convex mirror ( 3 ), the laser radiation widening such that it is conical as a symmetrical to the optical axis ( 5 ) cone-shaped bundle of rays ( 21 ) the convex mirror ( 3 ) strikes, and the cone angle is selected so that the laser radiation after passing through the Cassegrain mirror lens ( 3 , 4 ) is focused on the punctiform area ( 1 ). 4. Raman microscope according to claim 3, characterized in that the semitransparent plane mirror ( 6 ) is transmissive for Raman radiation in an observed wavelength range and reflective for the irradiated IR laser radiation. 5. Raman microscope according to claim 2, characterized in that the laser radiation as a parallel beam with a small beam diameter via a fully reflecting plane mirror ( 7 ) with lateral offset to the optical axis ( 5 ) and on the sample-facing side of the Casse grain Mirror lens ( 3 , 4 ) is arranged, is directed at the convex mirror ( 3 ) at such an angle with respect to the optical axis ( 5 ) that the parallel beam after passing through the Cassegrain mirror lens ( 3 , 4 ) on the punctiform area ( 1 ) hits. 6. Raman microscope according to claim 1, characterized in that the means for irradiating the laser radiation on the optical axis ( 5 ) between the test sample ( 2 ) and the Cassegrain mirror lens ( 3 , 4 ) arranged reflecting element ( 8 or . 13 ), with the help of which a substantially transverse to the optical axis ( 5 ) irradiated, parallel-focused laser beam can be directed along the optical axis ( 5 ) onto the punctiform area ( 1 ) on the upper surface of the test sample ( 2 ). 7. Raman microscope according to claim 6, characterized in that the reflecting element ( 8 or 13 ) has a smaller spatial dimension transverse to the optical axis ( 5 ) than the convex mirror ( 3 ) of the Cassegrain mirror lens ( 3 , 4 ). 8. Raman microscope according to claim 6 or 7, characterized in that the reflecting element ( 8 , 9 or 13 ) has a focusing effect, in particular the incident laser radiation on the punctiform region ( 1 ) on the upper surface of the test sample ( 2nd ) focused. 9. Raman microscope according to claim 8, characterized in that the reflecting element arranged at an angle to the optical axis ( 5 ) arranged plane mirror ( 8 ) and in the beam path of the laser radiation, preferably in the beam direction in front of the plane mirror ( 8 ) arranged focussing Includes lens ( 9 ). 10. Raman microscope according to claim 8, characterized in that the reflecting element is a preferably at 45 ° to the optical axis ( 5 ) arranged parabolic mirror ( 13 ).